HL7 Pass-through interface

For historic reasons we've got a mix of ADT feeds coming out of our PAS (TrakCare) to a wide range of downstream systems. In particular, there are some that are direct from TrakCare to the downstream systems, and many more that pass through Ensemble as our integration engine.

This is complicating management of the integrations, and so we'd like everything to go through the integration engine. In other words move from the flow in the top of the diagram to the flow in the bottom of the diagram:

So we want to build a couple of pass-through interfaces in Ensemble that respond identically to the current direct feeds - no transformations, and all ACK behaviour is transparent so that the only change visible to the TrakCare PAS is a change of IP/port for each of the currently direct feeds.

Should be easy, right? An HL7 TCP Service, possibly a routing process that simply passes everything on, and an HL7 TCP Operation that is connected to the downstream...

But what is happening is that the Router Process is generating an ACK and sending it back to the Service before the ACK comes back from the downstream. (I've faked a downstream, manually populated the ACK its sending back so I can definitely identify it. I can see it arriving in Ensemble - but after an ACK is sent to the upstream via the Service.) See the annotated message trace below:

Any pointers to what to be looking at?

I'm aware of Important HL7 Scenarios | Routing HL7 Version 2 Messages in Productions | InterSystems IRIS for Health 2025.1 and the Settings for HL7 Services, linked from there, but not seeing what we might have set wrong...

Thanks, Colin